1. Field of the Invention
The present invention relates to chemical analysis using mass spectrometers, and in particular to mass spectrometers using a plasma and an electrospray ionization source.
2. Related Art
Mass spectrometers and other systems are used for measurement of the concentration of analytes or the detection and measurement of contaminants and trace additives in solutions and gases. As one example in the field of semiconductor processing, process solutions for wafer cleaning, etching and other forms of surface preparation are routinely analyzed using mass spectrometers with plasma ionization sources, one type is an inductively coupled plasma mass spectrometer (ICP-MS). The measurements made by ICP-MS are used to determine and manage the quality of process solutions. Ultrapure water (UPW), dilute hydrofluoric acid (HF), and standard industry clean formulations SC1 (Standard Clean 1, ammonium hydroxide and hydrogen peroxide in water) and SC2 (hydrochloric acid and hydrogen peroxide in water) are examples of solutions that are routinely analyzed. Quick and accurate analysis in these and other industrial processes can result in the early detection of contamination problems, better control of process chemistry, and ultimately lead to higher yields and less product variation.
In general, mass spectrometry is often used to achieve sensitivity of parts per billion (ppb) or parts per trillion (ppt). It is commonly used to quantitatively measure the amount of contamination present or the concentration of a constituent in the solution. For example, commonly-assigned U.S. patent application Ser. No. 10/004,627, which is incorporated by reference in its entirety, discloses an automated analytical apparatus measuring contaminants or constituents present in trace concentrations using, for example, an electrospray ionization source. For such an in process mass spectrometry (IPMS) technique, a sample of interest is spiked with a known amount of spike. A resulting ratio measurement determines the concentration of the chemical constituents of interest in the sample.
Two modes for analyzing samples are used in the analysis method of this patent application: speciation mode and elemental mode. These modes are enabled by an electrospray ionization source. For applications that require molecular information, an electrospray ionization source is often used, such as disclosed in U.S. Pat. No. 6,060,705 entitled “Electrospray and Atmospheric Pressure Chemical Ionization Sources”, which is incorporated by reference in its entirety. This type of source provides a “soft” ionization (i.e., occurring at lower energy) in which molecular information is retained. This information is required for the successful identification of organics and molecular complexes that may be present in a process solution or gas. In speciation mode, collisions between the ions and other molecules are relatively soft, leaving the majority or major fractions of the structure of the original molecule intact.
On the other hand, in elemental mode, the collisions are much more energetic (“harder”) through the creation of more highly accelerated ions (with higher energy) that break the molecular species into their elemental or individual atomic components. However, the energetics present in the electrospray ionization source are not sufficient to break all components of the molecular species that may be present into their elemental components even in the hard ionization mode. The elemental sensitivity when using this type of source is limited by the fact that elemental species are distributed in a number of molecular fragments even after ionization. In this case, all peaks containing the element must be identified and analyzed after background subtraction if the optimum sensitivity is to be obtained. If, however, the analyte is fully ionized to its elemental components, an elemental ion of a given type will be concentrated into one peak that is relatively easy to identify and analyze without the errors associated with multiple peak fittings and background subtractions that must occur for the former case.
Another shortcoming of the electrospray source is its degraded ionization efficiency for some species including metals in the presence of strongly acidic or basic solutions. This degradation significantly reduces the sensitivity for trace contamination and other constituents that are important for successful measurement of the analyte.
Therefore for elemental quantification and ultimate detection limits, an inductively coupled plasma (ICP) ionization source is often preferred due to its ability to completely break molecules into their elemental components. Strong acids and bases are also effectively neutralized in the plasma, another important feature. An ICP source works in general by coupling radio frequency (RF) energy into a gas stream containing the nebulized liquid or gas sample with the result that the sample is immediately heated to several thousand degrees. Molecules break apart at these temperatures and collision energies leaving only elemental ions. Since this technique breaks all of the molecular bonds, this ionization technique can provide very high elemental sensitivity; however, all molecular information is lost. ICP sources that are currently available for sample ionization are too large and intrusive for successful integration into current electrospray mass spectrometry systems.
Another way to generate plasma for ionization purposes is with the use of a microwave induced plasma (MIP) source. It is well known that microwave energy, a higher frequency radiation than that used in ICP-MS instruments, is capable of inducing plasma that can successfully ionize analytes into elemental components for mass spectrometry analysis. There is extensive discussion of prior art in U.S. Pat. No. 5,051,557, entitled “Microwave Induced Plasma Torch with Tantalum Injector Probe” by Stazger and in an article by Yongxuan Su, Yixiang Duan and Zhe Jin entitled “Helium Plasma Source Time-of-Flight Mass Spectrometry: Off-Cone Sampling for Elemental Analysis,” published in Analytical Chemistry, Vo. 72, No. 11, Jun. 1, 2000, pp. 2455-2462. Both are incorporated by reference in their entirety.
A microwave source, due to its shorter wavelength, can be made significantly smaller than commercially available ICP sources normally used in mass spectrometry. The smaller size makes its integration into an electrospray ionization source mass spectrometer instrument possible while keeping the electrospray source operational as an alternative ionization source, i.e., the mass spectrometer can then be operated with an electrospray ion source or a microwave induced plasma ion source or a combination of the two.
For many applications, such as the measurement and control of semiconductor cleaning baths or processing gases, the ability to analyze for organics and species as well as high elemental sensitivity is highly desirable. Metals incorporated into semiconductor devices can affect device parameters, reliability, and yield. Knowing the oxidation state or molecular binding provides root cause source information. Organics deposited on wafer surfaces can affect transistor gate oxide thickness control and gate oxide reliability. It is desirable to have as low a detection limit as possible for metal contaminants while still having the ability to analyze molecular species present in process solutions.
Therefore, there is a need for a mass spectrometer system that overcomes the deficiencies as discussed above with conventional systems.